We propose to conduct a randomized double-blind placebo controlled, two by two factorial design clinical trial of the efficacy of nutritional supplements in preventing adenomatous polyps of the large bowel in persons at high risk for this condition. This is a collaborative investigation to be conducted at six study centers with a central data coordinating center. Eligible subjects will have had an adenomatous polyp removed within three months of recruitment. Study participants will be assigned at random to one of four treatment groups: (a) 25mg of beta carotene per day, (b) one gram of Vitamin C (ascorbic acid) and 400 mg of Vitamin E (alpha tocopherol) per day, (c) 25mg of beta carotene, one gram of Vitamin C, and 400 mg of Vitamin E per day, and (d) a placebo regimen. They will be monitored for toxicity and compliance to the drug regimen at six monthly intervals throughout the study. At twelve months and 36 months after study entry subjects will receive a complete colonoscopy with removal and microscopic examination of all polypoid lesions. Blood will be drawn and analyzed for beta carotene, retinol, alpha tocopherol, zinc, selenium, and retinol binding protein. The study is designed to detect a 35% reduction in polyp incidence attributable to either beta carotene or Vitamin C and E supplementation. It will also permit an investigation of possible synergistic effects between the carotenoid and antioxidant therapies.